Peltz's hedge fund Trian Fund Management, a major DuPont shareholder, has for months criticized the company's performance. Trian launched a proxy battle in January for four board seats, including one for Peltz. DuPont has said it could accommodate one of Trian's nominees, but not Peltz himself.

ISS recommended voting for two of Trian's four nominees to the DuPont board, and that shareholders do not vote for management's proxy card.

"We are very pleased with the ISS endorsement. ISS agrees with us that the company's operating performance is not what it should be and that change is needed in the Boardroom," Peltz wrote in a note to CNBC.

"Furthermore, ISS believes that having a Trian representative on the Board will add a strong, well-informed and constructive voice for shareholders," he added.

"This board has a long record of underperformance," Peltz said at an investor meet earlier this month, adding that it was time for "adult supervision" on the board.

The proxy battle will likely come to a head on May 13, when DuPont holds its annual shareholder meeting.

—CNBC's Mary Thompson and Scott Wapner, and Reuters contributed to this report.